Method for making dental coping

ABSTRACT

A method for making precision fixed crown and/or bridgework utilizing a non-gold base metal. More specifically, a method for casting a fixed crown and/or fixed bridgework from chrome cobalt and chrome nickel alloy metals heretofore employed only in the construction of bridgework not involving dental crowns or copings (i.e., removable bridgework). The present method basically involves preparing and heating to certain specific temperatures the crown or coping mold, then heating the alloy base metal and centrifugally casting the alloy into the mold whereby such hitherto unsuitable and less expensive alloys can now be employed to cast fixed crowns and/or bridgework.

United States Patent Scott [45] Aug. 22, 1972 METHOD FOR MAKING DENTAL COPING A [72] Inventor: Warren J. Scott, 130 l-lesper Ave:,

Metairie, La. 70005 [22] Filed: May 25, 1970 [21] Appl. No.: 40,418

[52] US. Cl. ..29/l60.6, 29/527.6, 32/2, 32/12, 164/34 [51] Int. Cl ..B2lf 43/00, B23p 13/00 [58] Field of Search ..l64/34, 45, 289; 29/l60.6, 29/460, 527.6; 264/17, 18; 32/2, 12

[56] References Cited UNITED STATES PATENTS 3,192,583 7/1965 Fryrear ..l64/34 3,258,509 6/1966 Bamhart ..264/l7 1,519,969 12/1924 'BeChtOld ..32 12 Primary Examiner-John F. Campbell Assistant ExaminerD0nald C. Reiley Attorney-Calvin J. Laiche [57] ABSTRACT The present method basically involves preparing and heating to certain specific temperatures the crown or coping mold, then heating the alloy base metal and centrifugally casting the alloy into the mold whereby such hitherto unsuitable and less expensive alloys can now be employed to cast fixed crowns and/or bridgework.

10 Claims, No Drawings METHOD FOR MAKING DENTAL COPING BACKGROUND OF THE INVENTION While there are numerous dental alloys on the market today, gold, gold based alloys, or other precious metal alloys having closely related physical properties are principally employed in the construction of fixed crown and/or bridgework. This is primarily due to the excellent properties that gold and gold base alloys possess which makes that metal emminently suitable as a dental material. Other precious metals and alloys thereof are also employed, however, they are generally tailored to resemble or duplicate the working qualities of gold and its alloys. Thus, the main thrust in the art has been in the use of gold and accordingly, dental equipment and supplies have been perfected to enable the realization of excellent results in the construction of fixed crown and fixed bridgework.

Needless to say because gold and other closely related precious metals (with reference to physical properties) are costly, coupled with certain physical limitations, e. g., softness, ductility, and the like, efforts have always been made in the art to provide less expensive 7 and stronger base metals.

A key requirement of these other metals is that they have a melting temperature approximating that of gold. Exemplary of such prior art is the metal alloy system as disclosed in US. Pat. No. 3,121,629. That patent provides an alloy preferably comprising chromium, nickel, and platinum, plus miscellaneious other constituents in minor amounts. The fusion temperature of that system ranges between 2,200 F and 2,350 F which is only slightly higher than that of gold alloys, generally around 2,000 F. That patent typlifies the thrust of the prior art toward non-gold metal alloys systems which are tailored to have a fusing temperature as close as possible to that of gold. Otherwise, these alloys would not be suitable for use in commonly employed techniques for making fixed crown and fixed bridgework utilizing gold and gold based alloys. The technique for making such appliances is generally describes as the lost wax method.

Needless to say, there are many alloy systems which would be less expensive and in many instances, offer much better physical properties. However, these alloys have much higher fusing temperatures and accordingly, are not adaptable for fabricating fixed crown and fixed bridgework using the lost wax method employing conventional dental equipment and supplies. One such alloy system is a chromium cobalt which has a fusing temperature of about 3,250 F. Another such system, lesser preferred, is chromium nickel which has a fusing temperature of about 2,650 F. For this reason, these high melting temperature alloys are not employed today to construct fixed crown and fixed bridgework even though they are not only less expensive than gold, but additionally possess many superior physical properties, especially rigidity. By way of the present invention, a method has been found to cast these alloys having fusing temperatures higher than that of gold while still utilizing the conventional lost wax methods and equipment, normally ascribed to the specific requirements of fixed crown and bridgework, but with certain variations.

SUMMARY OF THE INVENTION The present invention is directed to a method for making a fixed crown, bridge, or inlay. Initially, the dentist grinds a patients teeth to receive the fixed crown or bridge. Thereafter, an impression is made wherefrom the desired fixed crown or bridge is made pursuant to the present technique. First, an artificial stone model of the teeth is made from the impression. Second, removable dies are made from the stone model, the dies dimensionally duplicating those teeth having been ground to receive the fixed crown or bridge. Third, a wax pattern of the desired configuration of the crown, bridge, or inlay is constructed. The pattern includes extended wax portions (called sprues) which form channels whereby the wax is removed by melting it'out and thereafter serve for injecting metal into the mold as constructed in the next step. Fourth, a mold of the wax crown pattern is made by imbedding the pattern in an investment material and thereafter burning out the wax pattern by heating the material to a temperature within the range of about 1450 F to about 1800 F. thereby leaving a cavity in the mold of the desired configuration. Fifth, the alloy metal is then centrifugally cast into the cavity defined in the mold which forms the desired configuration out of the alloy metal by first heating the metal within a certain temperature range and thereafter centrifugally flowing the metal into the mold. Sixth, the formed metal configuration is recovered from the investment. Seventh, the formed metal configuration is then cleaned and polished. Eighth, the normally visible teeth portion of the metal configuration is faced with a plastic or porcelain material, the latter being optional in accordance with the dental prescription.

PREFERRED EMBODIMENT The present process for fabrication of fixed crown, coping, inlay, or bridge using a chrome-cobalt or chrome nickel alloy is similar in some respects to those techniques employed today which utilize a wide assortment of gold and other precious metal alloys. These techniques include the construction of full cast crowns, simple and complex inlays, veneer crowns, and bridge pontics for facing with thermoplastics, or full coverage with porcelain.

Preparatory to utilizing the present technique, a model of a patients teeth is first made in the customary manner by a dentist. This comprises preparing the teeth to receive the fixed crown or inlay by reducing those teeth to the desired configuration. This is generally accomplished by removing a portion of the tooth enamel, generally employing a high speed hand piece with a diamond cutting tip, or burr as they are commonly called.

The dentist then makes an impression of the altered or ground teeth and the surrounding area depending upon the extent of the dental restoration to be made. For this purpose, plastic rubber like material is generally employed. This material is then placed in an impression tray, inserted in the patients mouth, and pressed into the desired area. In this manner, an impression of the area to receive the dental restoration is made in the material which after setting is removed from the patients mouth. The impression is then generally sent to a dental laboratory for construction of the dental restoration.

Employing the impression as made above, the dental laboratory technician, using a gypsum based material which is also commonly referred to as artificial stone, makes a model of the tooth structure to receive the dental restoration. This is preferably accomplished in two steps. First, the artificial stone is poured into the impression up to a level or depth sufficient to define only the teeth portion of the mold, that is, up to the gum area. Thereafter, but before hardening, tapered brass dowel pins are inserted, taper end up, into the individual areas representing the teeth to be worked on. These dowel pins serve as a positioning means for the individual die or jig made in the manner set forth below.

Second, after the tooth portion has dried or hardened, with the tapered ends of the dowel pins projecting up, the remainder of the model is made by pouring additional artificial stone into the impression. The latter material is generally somewhat a softer type of gypsum material which is usually of a different color to clearly define the gum and teeth areas of the completed model. The second mix is poured over the brass dowel pins and allowed to set for at least 1 hour.

After setting, the model made in the above manner is then removed from the impression, the latter is usually destroyed in the process. Dies of the ground teeth are thereafter made by cutting through the areas adjacent the ground teeth to the depth of the gum area or interface defined between the two different types of artificial stone employed in the make-up of the model. This is usually accomplished by way of a wafer thin emery cutting disc or jewelers saw passed alongside a respective ground tooth along a plane essentially perpendicular, so to speak, to the radial or curved center line of the teeth. The dowel pin is then tapped from its bottom side to facilitate removable of the die. The die is then trimmed to its fine margins originally defined by the dentist by his grinding of the respective tooth. This margin is generally made at the base of the tooth a slight distance below the gum area whereby the subsequent metal coping fabricated pursuant to the present invention is fixed in well known customary manners.

The die made pursuant to the above procedure is then lubricated and waxed to the desired tooth shape or configuration. In this manner, a wax crown pattern of the final tooth form is achieved which is employed to make a duplicate metal structure which either alone or when faced with a plastic or porcelain material, forms a dental coping that at least essentially duplicates the patients original tooth (or teeth).

The wax crown pattern is then sprued, that is, an extended wax portion is attached to the wax pattern which portion serves as a channel for casting the pattern in the manner as set forth hereinafter. A surprising feature of the present invention is that it has been found that by attaching the sprue to the wax pattern in a particular manner depending upon the configuration being cast, it is possible to form copings, crowns, bridges, or inlays out of chrome cobalt and chrome nicel alloys heretofore found only to be possible when utilizing gold alloys and other similar precious alloys.

For example, when casting a single posterior full crown, an essentially straight piece of about 10 gauge round sprue wax, approximately 3 mm in length, would be employed. The sprue is attached to the wax pattern at the thickest area. By posterior full crown, according to contemporary usage, is meant an all metal dental restoration or device intended to supplant, in form an dimension, the crown or visable portion of a natural tooth which can be attached to one or more of the last five teeth, right or left, reposing astride the maxillary or mandibular osteological promininences which basically form the human oral cavity. It has thus been found that when employing a sprue as above designed, an excellent full crown can be cast of posterior teeth. Such teeth are characterized as being bulky and difficult to cast when employing other conventional methods of spruing due to shrinkage porosity (i.e., voids or pitted surfaces) endemic to results obtained with other methods of spruing.

When casting inlays and Y4crowns, a sprue formed into a 360 horizontal loop, or pigtail, is preferably employed. For normal castings, 10 gauge round sprue wax is employed which is attached to the thichest area of the casting. For smaller castings, 12 gauge or 14 gauge round sprue wax is employed. The overall length of the sprue should not exceed about 12 mm.

For constructing porcelain fired on cobalt, whether they be single anterior (six front teeth) or posterior (rear five teeth on each side) the design of sprue employed above for inlays and %crowns is employed. On single anterior crowns, the sprue is attached to the apex of the labial lingual planes or tip of incisal. On single posterior, the sprue is attached to the apex of the lingual occlusal, equidistant from the mesial to distal at the incisal edge at an inclination of about 45 to the occlusal plane.

When constructing multiple unit chrome cobalt or chrome nickel alloy restorations which would include abutrnents and pontics, full coverage porcelain veneer or thermoplastic, or porcelain labail buccal veneer, with waxed up bridge pattern in place on a model, a multiple of sprues or employed. The sprues are preferably made from 10 gauge sprue wax of 3 mm lengths. On anterior abutment or pontic copings, the sprue is attached at the incisal apex with about 20 lingual inclination. On posterior units of either the full coverage or cast occlusal type, the sprue is attached at the intersection of the lingual occlusal planes with about a 45 linqua] inclination, being equidistant from the mesial distal segments. These sprues are called final leads. The lingually inclined ends of the final leads are then tied together with a single length of 10 gauge round sprue wax which is fonned in a straight or arched line, more or less parallel to the longitudinal geometry of the bridge or framework. This sprue line is commonly called the base feed. It is important that the base feed line be positioned essentially at right angles to each individual final lead, the latter being the leads to each tooth of the structure. Failure to observe this requirement will usually result in poor or rounded margins and short or nonexistant areas in thin portions of veneers and copings. At successive points on the base feed line, precisely in the middle and between the final leads, 10 gauge round sprue wax of appropriate lengths is utilized to form an arch an a convergence at a point approximately at the center of the work. This portion of the sprue structure is called the primary lead. These leads should approach their juncture with the base feed line at exactly a 90 angle. At this point, the primary leads are sealed together, the work lifted from the model, contact points built out, and then sealed to an appropriate sprue former. A sprue former is usually constructed in a circular design with an extended portion that protrudes downward in the investment assembly so as to provide a depressed or concave area in the top of the completed investment which serves as an entrance for the metal during the casting step set forth below. The sprue of the wax pattern is attached to the sprue former at the apex of the extended portion.

When constructing a two unit casting, a single primary lead is employed. The lead is thickened somewhat with inlay wax to provide extra strength for handling while investing. The total distance from the casting surface to the tip of the sprue cone or extended portion should be from 10 to 13 mm depending upon the number and arrangement of units comprising the casting being constructed.

After constructing a wax pattern on the die and spruing it pursuant to the above procedures, an investment mold is made from which the metal casting is in turn to be made. The investment material is preferably a phosphate bound silica material. An investment ring of a suitable size is prepared by lining it with asbestos paper and sealing both ends and the seam. The ring is placed upon a flat surface and then filled with the investment by pouring into the open upper end. The waxed pattern is first dipped in a debubblelizing solution to relieve surface tension on the wax, care being taken to insure that the solution wets all of the crevices in the pattern. Using a semi-stiff brush, the wax pattern is next coated with some of the investment material. Care must be taken to be sure of filling all of the abutments and entire surface of the pattern. The investment coated pattern is thereafter inserted into the investment filled ring by shaking gently and carefully until the sprue former comes into contact with the edge of the ring. The assembly is allowed to set until hard for 45 minutes to 1 hour.

After the investment has hardened and the sprue former removed, the assembly is cleaned with an air hose or soft brush to remove loose particles of investment from around entrances to the primary leads. it is then placed in a furnace preheated to about 200 F to 300 F with the spruehole down. The temperature is then brought up to about 1,450 F to about l,550 F, preferably around 1,500 F, in to 40 minutes. After that period of time, the assembly is inverted and allowed to heat soak within the furnace for at least another 30 minutes in the case of small castings, and for about 60 minutes for larger castings.

The next step in the present technique is to cast the desired dental configuration that has been defined in the investment, employing c-chrome cobalt or chrome nickel alloy. Prior to the present technique, these relatively high temperature melting alloys would not have been thought to be useful for casting the broad range of dental configurations that are that are realized by way of the present invention, yet alone cast such configurations in a phosphate bound silica investment material which has only heretofore been employed for gold casting.

Suitable equipment for casting comprises a standard spring drive casting base having a heavy duty spring for rotating a crucible holder arm assembly rotatably mounted on the top of the base. The arm assembly to which is fitted a standard large ceramic crucible. The crucible and arm assembly is initially wound to five full turns. The heat source is a conventional oxy-acetylene torch. In operation, the aceteylene regulator is set to a pressure of about-6 lbs. per sq.in. and the oxygen regulator to about 12 lbs. per sq.in. The torch is lighted and the flame adjusted until a neutral burn is achieved, that is, it is neither carbonizing nor oxidizing. Such a flame is characterized as having a clearly delineated bright blue cone with an iridesecent glow in the center of about 1; in. long. The alloy is placed in the crucible and melted with the torch by bringing the bright cone portion of the flame to within an inch of the metal surface. The flame is played on the surface of the alloy in a more or less circular motion, favoring to some degree, the larger ingot which normally would be the reservoir portion or button from a previous casting in order to bring the two segments to a point of collapse simultaneously. The color of the alloy during-this period should be the determing factor as to correct heat distribution (this consideration is-importantto avoid burning the lower fusing alloys within the smaller mass). The fusing point is reached when the ingots begin to round off and tend to settle to the bottom of the crucible.At this time, as assistant inserts the ring mold into the cradle while the operator continues the melting process until the alloy has formed into an oval shaped mass in the bottom of the crucible. Following this, one or two close passes are made around the metal and the machine is released and allowed to spin out before removing the ring mold from the cradle.

When employing a chrome cobalt alloy metal, the metal is heated to a temperature within the range of about 2,900 F to about 3,400 F, preferably between 3,200 F to 3,300 F, and especially around 3,250 F. On the other hand, when employing a chrome nickel alloy metal, the metal is heated to a temperature within the range of about 2,500 F to about 2,900 F, preferably between 2,600 F to 2,7 00 F, and especially around 2,650 F. The exact temperature is, however, actually determined by the judgment of the operator applying the above criteria, and in essence, is mainly controlled by the structure of the particular alloy is being employed.

The color of the alloy at the time of actual casting should be slightly less than incandescent (i.e., to the operator, wearing protective lenses) the surface having a rather uniform mat appearance with no broken areas of shiny metal showing through. The mat look is formed by oxidation on the metal surface. Its intactness is desirable from the standpoint of maintaining the cleanliness of the melt and, to some extent, thermally controlled. When it is broken, carbon particles may be introduced into the alloy, causing it to become extremely hard and brittle. Overheating of the alloy is observed when the molten alloy displays an agitated condition with broken patches in the oxide shell. Bright sparks may also be seen emitting upward and outward from the crucible. This is usually brought about by crowding the melt and holding the tip too close to the alloy after it has reached a molten state. This is corrected by moving the tip back slightly until the normal condition reappears. Over heated or burned castings are identified by porous ragged buttons with bubbles and crater-like irregularities on the crucible side surface which, itself, may exhibit a somewhat convex appearance. The optimum results in casting, regarding the appearance of the visible surface of the button, is evidenced by sharp edges and a slightly concave smooth surface with a very fine crinkle grain, coated by a dull black oxide.

The casting may be allowed to bench cool or may be quenched once the button has lost its red glow, by holding in ring pliers, investment end up, beneath running tap water. Avoid submersing immediately since to do so may cause explosion of still red hot investment inside. Once cooled, the casting is removed from the ring. The bulk of the investment is knocked off by tapping the button with a tack hammer or the edge of a bench scraper. Following this, the casting is sand blasted lightly to remove any remaining material from the outer surfaces. Since the investment is of a very hard type, removal of core from inside abutments will be found to be difficult. This is best accomplished by holding it against a vibrating surface and using a carbide tipped pick against the investment remaining on the casting. Sandblast again thoroughly, both in and out. Prolonged sandblasting of the inner surfaces will result in badly rounded off margins on inlays and abutments and hence should be strictly avoided. The casting is then ready for despruing and finishing.

Finishing can be accomplished in customary methods as applied to finishing golds. Chrome cobalt and chrome nickel alloys are, of course, considerably harder than gold alloys. Herein lies the greatest difference insofar as finishing is concerned. Where burs made of carbon steel are indicated for use in finishing gold, carbide-tipped burs must be substituted and coupled with'a turbine hand piece and standard finishing lathe with changeable ratio belt drive for supplying the more desirable higher r.p.m. necessary for efficient finishing and polishing. Cobalt and nickel alloy lends themselves very well to the practice of pulling or bumishing margins of abutments and inlays when done in the usual way.

The above casting is then ready for facing where desired assuming the casting does not duplicate the outer final tooth configuration of the dental reconstruction being fabricated. That is, castings made pursuant to the above procedure that duplicate the final tooth structure (which would generally only comprise rear teeth) are not intended for facing. On the other hand, frontal teeth restorations are invariably faced with either a thermoplastic or porcelain. Among the distinct features and advantages of the present invention is the fact that castings made by way of this technique can be readily faced with porcelain more efficaciously than can conventional gold alloy castings employing prior art procedures.

I claim:

1. In a method for making a fixed precision dental restoration comprising either a fixed'crown, coping, inlay, or bridge from an impression of teeth having first been ground to receive a fixed dental restoration constructed of an alloy base metal and on which a tooth simulating porcelain or plastic jacket may be sub sequently applied, which method comprises making an artificial stone model of the teeth from the impression; making removable dies from the stone model simulating those teeth that have been ground to receive the fixed dental restoration; making a wax crown pattern having the desired configuration of the dental restoration, to which pattern a sprue is attached for removal of the wax forming the wax crown pattern and which also subsequently serves as a channel by which metal is centrifugally cast into a mold of the wax crown pattern; making a mold of the wax crown pattern; centrifugally casting a chrome cobalt alloy into the cavity defined in the mold; recovering the formed metal configuration; cleaning and polishing the formed metal configuration; and facing with porcelain the portions of those teeth having been ground to receive a plastic or porcelain material; the improvement for casting a single posterior full crown restoration comprising: attaching the sprue to the wax crown pattern at the thickest area of the pattern.

2. The method of claim 1 further characterized in that said sprue is formed of an essentially straight piece of about 10 gauge round sprue wax approximately 3 mm in length.

3. Ina method for making a fixed precision dental restoration comprising either a fixed crown, coping, inlay, or bridge from an impression of teeth having first been ground to receive a fixed dental restoration constructed of an alloy base metal and on which a tooth simulating porcelain or plastic jacket may be subsequently applied, which method comprises making an artificial stone model of the teeth from the impression; making removable dies from the stone model simulating those teeth that have been ground to receive the fixed dental restoration; making a wax crown pattern having the desired configuration of the dental restoration, to which pattern a sprue is attached for removal of the wax forming the wax crown pattern and which also subsequently serves as a channel by which metal is centrifugally cast into a mold of the wax crown pattern; making a mold of the wax crown pattern; centrifugally casting a chrome cobalt alloy into the cavity defined in the mold; recovering the formed metal configuration; cleaning and polishing the formed metal configuration; and facing with porcelain the portions of those teeth having been ground to receive a plastic or porcelain material; the improvement for casting inlays and 54 crowns comprising attaching a sprue formed into a loop to the wax crown pattern at the thickest area of the pattern.

4. The method of claim 1 further characterized in that said sprue is formed into a 360 horizontal loop of about 10 gauge to about 14 gauge round sprue wax of less than about 12 mm in length.

5. In a method for making a fixed precision dental restoration comprising either a fixed crown, coping, inlay, or bridge from an impression of teeth having first been ground to receive a fixed dental restoration constructed of an alloy base metal and on which a tooth simulating porcelain or plastic jacket may be subsequently applied, which method comprises making an artificial stone model of the teeth from the impression; making removable dies from the stone model simulating those teeth that have been ground to receive the fixed dental restoration; making a wax crown pattern having the desired configuration of the dental restoration, to which pattern a sprue is attached for removal of the wax forming the wax crown pattern and which also subsequently serves as a channel by which metal is centrifugally cast into a mold of the wax crown pattern; making a mold of the wax crown pattern; centrifugally casting a chrome cobalt alloy into the cavity defined in the mold; recovering the formed metal configuration; cleaning and polishing the formed metal configuration; and facing with porcelain the portions of those teeth having been ground to receive a plastic or porcelain material; the improvement for casting a porcelain fired on cobalt coping comprising attaching a sprue formed into a loop to the apex of the labial lingual planes when casting a single anterior crown and to the apex of the lingual occlusal when casting a single posterior crown, equidistant from the mesial to the distal at the incisal edge at an inclination of about 45 to the occlusal plane.

6. The method of claim further characterized in that said sprue is formed into a horizontal loop of about 360.

7. In a method for making a fixed precision dental restoration comprising either a fixed crown, coping, inlay, or bridge to each individual final lead; the primary leads being connected to the base feed sprue at essentially its mid-point, between the final leads, forming an arch and a convergence at a point approximately at the center of the work, from the lingual aspect, the primary leads forming a juncture with the base feed line at essentially a 90 angle; the primary leads being thereafter sealed together, the work lifted from the model, contact points built out, and then sealed to an appropriate sprue former constructed in a circular design with an extended portion protruding downward in the investment assembly, the sprue former being attached at the apex of its extended portion to the sprue of the wax pattern.

8. The method of claim 7 further characterized in that when said multiple unit restoration is a two unit casting, a single primary lead is employed with the total distance from the casting surface to the top of the sprue cone should be from about 10 to 13 mm.

9. In a method for making a fixed precision dental restoration comprising either a fixed crown, coping, inlay, or bridge from an impression of teeth having first been ground to receive a fixed dental restoration constructed of an alloy base metal and on which a tooth simulating porcelain or plastic jacket may be subsequently applied, which method comprises making an artificial stone model of the teeth from the impression; making removable dies from the stone model simulating those teeth that have been ground to receive the fixed dental restoration; making a wax crown pattern having the desired configuration of the dental restoracasting a chrome cobalt alloy into the cavity defined in the mold; recovering the formed metal from an impression of teeth having first been ground to receive a fixed jacket may be subseque ntly atpplied, w

dental restoration constructed of an alloy base metal and on which a tooth simulating porcelain or plastic hich meth comprises making an artr mm! s one mode of the tee from the impression; making removable dies from the stone model simulating those teeth that have been ground to receive the fixed dental restoration; making a wax crown pattern having the desired configuration of the dental restoration, to which pattern a sprue is attached for removal of the wax forming the wax crown pattern and which also subsequently serves as a channel by which metal is centrifugally cast into a mold of the wax crown'pattem; making a mold of the wax crown pattern; centrifugally casting a chrome cobalt alloy into the cavity defined in the mold; recovering the formed metal configuration; cleaning and polishing the vformed metal configuration; and facing with porcelain the portions of those 'teeth having been ground to receive a plastic or porcelain material; the improvement for casting multiple unit restorations including abutments and pontics, full coverage porcelain veneer or thermoplastic, or porcelain labial buccal veneer, by attaching a multiple of final lead, base feed, and primary lead sprues to said unit restorations, which in the case of anterior abutment or pontic copings, the final lead sprue is attached at the incisal apex with about a 20 lingual inclination, and in the case of posterior units of either the full coverage or cast occlusal type, the final lead sprue is attached at the intersection of the lingual occlusal planes with about a 45 lingual inclination, being equidistant from the mesial distal segments; the lingually inclined ends of the final lead sprues being tied together with a single length of base feed sprue wax formed in a straight or arched line essentially parallel to the longitudinal geometry of the bridge and positioned essentially at right angles configuration; cleaning and polishing the formed metal configuration; and facing with porcelain the portions of those teeth having been ground to receive a plastic or porcelain material; the improvement which comprises making said mold of the wax crown pattern out of a phosphate bound silica investment material.

10. The method of claim 9 further characterized in that said phosphate bound silica mold is heated to a temperature within the range of from about 1,45 0 F to about l,800 F. 

1. In a method for making a fixed precision dental restoration comprising either a fixed crown, coping, inlay, or bridge from an impression of teeth having first been ground to receive a fixed dental restoration constructed of an alloy base metal and on which a tooth simulating porcelain or plastic jacket may be subsequently applied, which method comprises making an artificial stone model of the teeth from the impression; making removable dies from the stone model simulating those teeth that have been ground to receive the fixed dental restoration; making a wax crown pattern having the desired configuration of the dental restoration, to which pattern a sprue is attached for removal of the wax forming the wax crown pattern and which also subsequently serves as a channel by which metal is centrifugally cast into a mold of the wax crown pattern; making a mold of the wax crown pattern; centrifugally casting a chrome cobalt alloy into the cavity defined in the mold; recovering the formed metal configuration; cleaning and polishing the formed metal configuration; and facing with porcelain the portions of those teeth having been ground to receive a plastic or porcelain material; the improvement for casting a single posterior full crown restoration comprising: attaching the sprue to the wax crown pattern at the thickest area of the pattern.
 2. The method of claim 1 further characterized in that said sprue is formed of an essentially straight piece of about 10 gauge round sprue wax approximately 3 mm in length.
 3. In a method for making a fixed precision dental restoration comprising either a fixed crown, coping, inlay, or bridge from an impression of teeth having first been ground to receive a fixed dental restoration constructed of an alloy base metal and on which a tooth simulating porcelain or plastic jacket may be subsequently applied, which method comprises making an artificial stone model of the teeth from the impression; making removable dies from the stone model simulating those teeth that have been ground to receive the fixed dental restoration; making a wax crown pattern having the desired configuration of the dental restoration, to which pattern a sprue is attached for removal of the wax forming the wax crown pattern and which also subsequently serves as a channel by which metal is centrifugally cast into a mold of the wax crown pattern; making a mold of the wax crown pattern; centrifugally casting a chrome cobalt alloy into the cavity defined in the mold; recovering the formed metal configuration; cleaning and polishing the formed metal configuration; and facing with porcelain the portions of those teeth having been ground to receive a plastic or porcelain material; the improvement for casting inlays and 3/4 crowns comprising attaching a sprue formed into a loop to the wax crown pattern at the thickest area of the pattern.
 4. The method of claim 1 further characterized in that said sprue is formed into a 360* horizontal loop of about 10 gauge to about 14 gauge round sprue wax of less than about 12 mm in length.
 5. In a method for making a fixed precision dental restoration comprising either a fixed crown, coping, inlay, or bRidge from an impression of teeth having first been ground to receive a fixed dental restoration constructed of an alloy base metal and on which a tooth simulating porcelain or plastic jacket may be subsequently applied, which method comprises making an artificial stone model of the teeth from the impression; making removable dies from the stone model simulating those teeth that have been ground to receive the fixed dental restoration; making a wax crown pattern having the desired configuration of the dental restoration, to which pattern a sprue is attached for removal of the wax forming the wax crown pattern and which also subsequently serves as a channel by which metal is centrifugally cast into a mold of the wax crown pattern; making a mold of the wax crown pattern; centrifugally casting a chrome cobalt alloy into the cavity defined in the mold; recovering the formed metal configuration; cleaning and polishing the formed metal configuration; and facing with porcelain the portions of those teeth having been ground to receive a plastic or porcelain material; the improvement for casting a porcelain fired on cobalt coping comprising attaching a sprue formed into a loop to the apex of the labial lingual planes when casting a single anterior crown and to the apex of the lingual occlusal when casting a single posterior crown, equidistant from the mesial to the distal at the incisal edge at an inclination of about 45* to the occlusal plane.
 6. The method of claim 5 further characterized in that said sprue is formed into a horizontal loop of about 360*.
 7. In a method for making a fixed precision dental restoration comprising either a fixed crown, coping, inlay, or bridge from an impression of teeth having first been ground to receive a fixed dental restoration constructed of an alloy base metal and on which a tooth simulating porcelain or plastic jacket may be subsequently applied, which method comprises making an artifical stone model of the teeth from the impression; making removable dies from the stone model simulating those teeth that have been ground to receive the fixed dental resotration; making a wax crown pattern having the desired configuration of the dental restoration, to which pattern a sprue is attached for removal of the wax forming the wax crown pattern and which also subsequently serves as a channel by which metal is centrifugally cast into a mold of the wax crown pattern; making a mold of the wax crown pattern; centrifugally casting a chrome cobalt alloy into the cavity defined in the mold; recovering the formed metal configuration; cleaning and polishing the formed metal configuration; and facing with porcelain the portions of those teeth having been ground to receive a plastic or porcelain material; the improvement for casting multiple unit restorations including abutments and pontics, full coverage porcelain veener or thermoplastic, or porcelain labail buccal veener, by attaching a multiple of final lead, base feed, and primary lead sprues to said unit restorations, which in the case of anterior abutment or pontic copings, the final lead sprue is attached at the incisal apex with about a 20 degree lingual inclination, and in the case of posterior units of either the full coverage or cast occlusal type, the final lead sprue is attached at the intersection of the lingual occlusal planes with about a 45 degree lingual inclination, being equidistant from the mesial distal segments; the lingually inclined ends of the final lead sprues being tied together with a single length of base feed sprue wax formed in a straight or arched line essentially parallel to the longitudinal geometry of the bridge and positioned essentially at right angles to each individual final lead; the primary leads being connected to the base feed sprue at essentially its mid-point, between the final leads, forming an arch and a convergence at a point approximately at the center of the work, from the lingual aspect, the primary leads forming a juncture with the base feed line at essentially a 90 degree angle; the primary leads being thereafter sealed together, the work lifted from the model, contact points built out, and then sealed to an appropriate sprue former constructed in a circular design with an extended portion protruding downward in the investment assembly, the sprue former being attached at the apex of its extended portion to the sprue of the wax pattern.
 8. The method of claim 7 further characterized in that when said multiple unit restoration is a two unit casting, a single primary lead is employed with the total distance from the casting surface to the top of the sprue cone should be from about 10 to 13 mm.
 9. In a method for making a fixed precision dental restoration comprising either a fixed crown, coping, inlay, or bridge from an impression of teeth having first been ground to receive a fixed dental restoration constructed of an alloy base metal and on which a tooth simulating porcelain or plastic jacket may be subsequently applied, which method comprises making an artificial stone model of the teeth from the impression; making removable dies from the stone model simulating those teeth that have been ground to receive the fixed dental restoration; making a wax crown pattern having the desired configuration of the dental restoration, to which pattern a sprue is attached for removal of the wax forming the wax crown pattern and which also subsequently serves as a channel by which metal is centrifugally cast into a mold of the wax crown pattern; making a mold of the wax crown pattern; centrifugally casting a chrome cobalt alloy into the cavity defined in the mold; recovering the formed metal configuration; cleaning and polishing the formed metal configuration; and facing with porcelain the portions of those teeth having been ground to receive a plastic or porcelain material; the improvement which comprises making said mold of the wax crown pattern out of a phosphate bound silica investment material.
 10. The method of claim 9 further characterized in that said phosphate bound silica mold is heated to a temperature within the range of from about 1,450* F to about 1,800* F. 